1. Field of the Invention
The present invention relates to a vacuum enclosure having an X-ray window which is formed of vitreous carbon.
2. Description of the Prior Art
Vitreous carbon (glassy graphite), as is known from German OS 196 38 150, is suitable as a material for X-ray windows of vacuum tubes, e.g. X-ray tubes, but also X-ray image intensifiers, because it is nontoxic and is significantly less expensive than the materials which have been used hitherto, e.g. beryllium. The use of vitreous carbon as a material for X-ray windows of vacuum tubes, however, has in practice failed since it has been impossible to produce a sufficiently durable vacuum-tight joint between an X-ray window made from vitreous carbon and the materials, for example metal or ceramic, which are usually employed as material for vacuum enclosures of vacuum tubes.
The primary problem above all is that the solders which are customarily used in vacuum technology only wet vitreous carbon to an insufficient extent, and consequently a permanent vacuum-tight joint between the X-ray window and the vacuum casing is not guaranteed.
An object of the present invention is to provide a vacuum enclosure of the type described above having an improved joint between the X-ray window and the vacuum casing.
According to the invention, this object is achieved in a vacuum enclosure for a vacuum tube having an X-ray window which is formed of vitreous carbon and is joined to the vacuum casing by soldering using an active brazing alloy, the active brazing alloy preferably being a material which is marketed as Incusil ABA (Active Brazing Alloy) by Wesgo Metals, USA.
As used herein the term active brazing alloy means a brazing alloy which directly wets glassy graphite. The basis for the wetting is the formation of layers known as reaction layers which are formed by interface reaction in the brazing alloy-glassy graphite interface region. To enable these active brazing reaction layers to form, the brazing alloys which are to be used are doped with particularly reactive metals, such as for example Ti, Hf, Zr, Ns, etc. During the soldering, these active components diffuse at the interfaces and break down the graphite surface so as to form reaction products which in turn can be wetted by the brazing matrix. Due to the constituents of the brazing alloys, they are known as active brazing alloys.
Surprisingly, it has been found that active brazing alloys wet vitreous carbon without problems, generally without any special prior treatment of the vitreous carbon. Accordingly, in the case of the vacuum enclosure according to the invention, the X-ray window is joined to the vacuum enclosure permanently, in a vacuum-tight manner and, moreover, in an inexpensive way, namely by soldering.
A further problem, associated with the high temperatures which arise in particular for X-ray tubes, is that vitreous carbon has a coefficient of thermal expansion which, at 2.6xc3x9710xe2x88x926 xcexcm/xc2x0 C., is low compared to that of materials which are usually employed for vacuum casings, so that the risk of the joint breaking open is very high.
This is remedied, according to an embodiment of the invention, by providing a frame-like structure of tungsten or a tungsten alloy, to which the X-ray window is fitted in a vacuum-tight manner by means of the active brazing alloy. Tungsten or tungsten alloys have a coefficient of expansion which is approximately twice as large as that of vitreous carbon and changes to only an insignificant extent as a function of temperature. Consequently, the risk of the X-ray window being damaged as a result of the different coefficients of thermal expansion of the material of the vacuum enclosure and of the X-ray window is reduced.
In a further embodiment of the invention the vacuum enclosure also has a frame-like support of tungsten or a tungsten alloy, to which the frame-like structure provided with the X-ray window is fitted in a vacuum-tight manner by welding or soldering. Production of a vacuum enclosure of this type preferably ensues by first fitting the frame-like support to the vacuum enclosure, then the frame-like structure is fitted to the X-ray window. It is then possible to join the unit formed by the X-ray window and its frame-like structure to the vacuum enclosure provided with the frame-like support without problems in a vacuum-tight manner by conventional welding or soldering processes, since identical or similar materials, namely tungsten or tungsten alloys, are being soldered or welded to one another.
Suitable tungsten alloys are alloys marketed by Plansee under the trade name DENSIMET(copyright), which have tungsten contents of more than 90% and also contain nickel and iron or nickel and copper.